Shade leaves: nature’s most efficient light harvesters

Not all leaves are created equal. Within a single plant, a spectrum of light availability exists- some leaves are positioned to receive maximum sunlight every day, while others, by luck of the draw, are primarily shaded and experience full sun only in short, erratic bursts. The sun leaves, by virtue of their superior position, are equipped with the capacity to capture the most light and perform the most photosynthesis. They tend to have more cell layers, larger numbers of the enzymes involved in capturing CO2 and converting it to sugar, and a larger volume of space to perform carbon-fixing reactions than their less-exposed neighbors.

Though it may seem that shade leaves get a raw deal, they are in fact uniquely adapted to a low light environment. Seemingly minor biochemical changes allow shade leaves to be far more efficient light harvesters, maximizing the amount of sunlight captured during those rare moments of exposure.

Chlorophyll is the main light-harvesting pigment in most plants, capturing light radiation primarily in the red and blue parts of the visible spectrum. These light waves are between 400 and 700 nanometers in length and are known as photosynthetically active raditaion (PAR). When PAR is absorbed by chlorophyllous tissue, radiation of longer wavelengths in the 680-760 nanometer range is re-emitted in a process known as fluroescence.

Measuring the kinetics of chlorophyll fluorescence has proved a useful tool in discerning the different photosynthetic capabilities of sun and shade leaves. To do so, a plant physiologist may incubate identical leaves in contrasting light conditions and measured fluorescence levels when a “saturating” pulse of white light is applied in each case. It turns out that the dark incubated leaves respond far more dramatically to a brief pulse of light than that already light-exposed leaves.

The pulses of light used in such laboratory experiments are in fact comparable to the erratic “sun flecks” that understory leaves may experience over the course of a day. Much as a starving animal will metabolize and release energy more slowly than a well-fed one, shade leaves take the limited light resources available and make far more out of these brief, shining moments than sun leaves would ever be able to.

Thanks, Adrienne! To the best of my knowledge, this sun vs shade leaf distinction only applies to broadleaf and deciduous plants. There are several reasons why I don’t think it would apply to conifers. Conifers generally grow at high latitudes and receive less sunlight than lower latitude forests. Furthermore, the open, tiered architecture of conifer trees allows light to penetrate all the way to the lowest leaves. Coniferous forests tend to have less of an understory than deciduous forests, further diminishing the difference between upper and lower leaves.

What all this basically sums up to is that conifers are generally more light limited than deciduous trees, but are constructed in such a way that their leaves all have relatively similar access to light, thus precluding the need for shade-leaf adaptations. I’ll dig around for a study or two to back this up, I’m a bit curious now.

Another interesting facet of leaf photosynthesis is that leaves generally require time in the dark to “rest” their photosynthetic machinery. Over-saturation with light can damage light-harvesting molecules and plants in very high light environments have mechanisms for coping with this. One reason the tree line exists where it does in the northern hemisphere is that as you get to very high latitudes, you get a situation where light is available for months at a time, which most plants can’t handle!

They do make sugar in leaves- when light is captured by chlorophyll, it excites electrons that provide energy to other electron carriers. These are sent out of the chlorophyll to another part of the leaf, where their energy is used to fix CO2 into sugars. I thin sun leaves generally do produce more sugar by virtue of the fact that they have so much more access to light. But per photon of light, shade leaves are far more efficient.

Wait, are you saying that for circum-polar trees(/plants?) getting too much light in the ~6 months of day is a bigger challenge than getting no light in the ~6 months of night? That seems counter-intuitive (though I don’t have any special knowledge about this).

You basically got it. When more energy is entering a chloroplast than its photosystems can absorb, excess energy can give rise to the production of reactive oxygen species and radicals that break down membranes and chlorophyll. This is called photodamage. In fact, an entire biochemical pathway intended to prevent photodamage, known as the xanthophyll cycle, can be activated in plants that experience high-light environments. This cycle involves pigments that, in response to excess light, induce conformational changes in light-harvesting photosystems which faciliate the dissipation of excess excitation energy.